The present invention relates generally to a display apparatus having an improved image quality. More particularly, the present invention relates to a display apparatus using electroluminescence elements.
Electroluminescence (EL) elements include an inorganic EL element which uses a thin film of an inorganic compound like selenium or zinc as a luminous material, and an organic EL element which uses an organic compound as a luminous material. The organic EL element preferably has the following features: (1) high luminous efficiency, (2) low drive voltage, (3) ability to display multifarious colors (green, red, blue, yellow, etc. by selection of a luminous material in use, (4) clear display and no need for a back light because it is of a self light emission type, (5) planar light emission and no dependency on the angle of visibility, (6) thin and light weight, and (7) low peak temperature in the fabrication process, which allows a soft material like a plastic film to be used for the substrate. Because of these properties, a display apparatus using such organic EL elements (organic EL display apparatus) has recently been getting attention as a replacement for a CRT or LCD.
An organic EL display apparatus can employ either a simple matrix system or an active matrix system. The simple matrix system allows an external drive unit to directly drive organic EL elements as a matrix of pixels, arranged on a display panel, in synchronism with a scan signal. Because the display panel of a display apparatus which employs this system is formed of organic EL elements, the drive time (duty) assigned to each pixel becomes shorter as the number of scan lines is increased, which lowers the contrast of an image displayed on the display panel.
The active matrix system has pixel driving element (active elements) provided respectively, for a matrix of pixels. Each pixel driving element serves as a switch which is switched on or off by the scan signal. When a pixel driving element is enabled, a data signal (display signal, video signal) is transmitted via that pixel driving element to an anode of the associated organic EL element and is written there. In this manner, the organic EL element is driven. When the pixel driving element is disabled, the data signal applied to the anode of the organic EL element is held as a charge in the organic EL element. The organic EL element is thus driven until the associated pixel driving element is switched on. The drive time per pixel becomes shorter as the number of scan lines is increased, therefore, the driving of the organic EL elements is not affected. As a result, the contrast is not lowered. In this respect, the active matrix system displays images with a higher quality than the simple matrix system.
The active matrix system employs transistor type (three-terminal type) pixel driving elements or diode type (two-terminal type) pixel driving elements. The transistor type is characterized as easily acquiring high contrast and high resolution, but is difficult to fabricate, as compared with the diode type. That is, the transistor type organic EL display apparatus provides high-quality images which match those displayed by a CRT. The operational principle of the active matrix system is mainly associated with the transistor type pixel driving elements.
Both the writing characteristic and the holding characteristic required of the individual pixels in the simple matrix system and active matrix system are important characteristics. The writing characteristic indicates whether or not a desired data signal can be written sufficiently in each organic EL element within a predetermined unit time conforming to the specifications of the display panel. The holding characteristic indicates whether or not a data signal, once written in each organic EL element, can be held for a predetermined time that conforms to the specifications of the display panel.
When multiple organic EL elements are arranged in a matrix, the size of the individual elements is structurally restricted and the electrostatic capacitance (hereinafter referred to as "capacitance") of the elements is limited. An organic EL element having a smaller capacitance has a lower holding characteristic, which makes it difficult to provide a display apparatus capable of displaying high-quality images.
Broadly speaking, the present invention is directed to a display apparatus using electroluminescence elements, which is capable of providing high-quality and stable display images. The present invention can be implemented in numerous ways, including as an apparatus and a method.